CT simulation is a specialized imaging procedure used to prepare a patient for radiation therapy. This preparatory step is a planning process, not a form of treatment, that creates a three-dimensional map of the patient’s internal anatomy. The goal is to precisely define the location of the tumor and the surrounding healthy organs that must be protected from radiation exposure. Acquiring this detailed geometric data forms the foundation for designing an accurate and personalized radiation treatment plan.
Distinguishing Diagnostic CT from CT Simulation
The CT simulation scan differs significantly from a standard diagnostic CT performed in a hospital’s radiology department. A diagnostic scan aims to find disease and evaluate its extent. Conversely, the simulation scan’s main function is to map the patient’s body geometry and create a reproducible setup for daily treatment.
Specialized CT scanners used for simulation often feature a wider opening, known as a “big bore,” to accommodate the patient along with any necessary immobilization equipment. The table, or couch, on a simulation scanner is flat and rigid, mimicking the treatment table on the actual radiation delivery machine, unlike the curved table found on most diagnostic scanners. The simulation room is equipped with fixed laser systems used for isocentric localization, which projects precise lines onto the patient to establish a reference point for accurate mapping. The data collected is also calibrated to convert CT numbers (Hounsells units) into electron density values, a technical requirement for the treatment planning software to accurately calculate how radiation will be absorbed by different tissues.
Patient Preparation for the Scan
Preparing for the simulation scan often involves specific instructions to ensure the internal organs are positioned exactly as they will be during the actual daily treatment sessions. This consistency is paramount, as even small shifts in organ location can affect the accuracy of the radiation delivery. For treatments targeting the abdomen or pelvis, patients may receive detailed instructions about their diet, hydration, and bowel clearance to manage the size and position of structures like the bladder and rectum.
A common instruction is to arrive with a full bladder, which helps displace the small bowel out of the treatment field for prostate or gynecological cancers. Bowel preparation, involving dietary restrictions and perhaps a mild laxative, may be required to empty the rectum, minimizing its movement and potential dose exposure. Patients may also be asked to fast if an intravenous contrast agent will be administered during the scan to better visualize blood vessels or the tumor itself. Following these preparatory steps ensures the internal anatomy captured accurately reflects the anatomy during every subsequent radiation treatment.
The Simulation Procedure
The simulation procedure begins with radiation therapists positioning the patient on the flat CT couch in the exact position that will be used for treatment. They first create or fit a custom immobilization device designed to restrict movement and ensure the patient’s position is identical each day. Depending on the area being treated, this might involve a personalized thermoplastic mask molded to the head and shoulders, a foam cradle, or a vacuum-sealed bag that conforms to the body and hardens when air is removed.
Once immobilized, therapists use the room’s fixed laser system to align the patient’s body to a precise coordinate system. These lasers project lines onto the patient’s skin or the immobilization device, establishing the isocenter—the central point used for planning the radiation beams. This meticulous setup process often takes the longest part of the appointment, ensuring all reference points are recorded before the scan begins.
The CT scan itself is relatively quick, capturing cross-sectional images of the treatment area while the patient remains perfectly still. After imaging, therapists place small, permanent skin markings (tattoos) or temporary ink marks on the patient’s skin. These tiny marks correspond to the reference points established by the lasers and are used daily by the treatment team to precisely align the patient to the radiation machine before each treatment session.
Translating Simulation Data into a Treatment Plan
Following the simulation, the acquired three-dimensional CT images are transferred to a specialized Treatment Planning System (TPS) software. A radiation oncologist then uses this digital data to begin contouring, which involves drawing boundaries on the images. The target volume is delineated first, defining the tumor and surrounding tissue that needs to receive the full radiation dose.
The oncologist also contours the Organs At Risk (OARs), which are healthy structures near the target that are sensitive to radiation, such as the spinal cord, heart, or lungs. This detailed mapping allows the medical dosimetrist, working with the radiation physicist, to calculate the exact radiation dose distribution. Using the TPS, they determine the optimal angles, shapes, and intensities of the radiation beams to ensure the target volume receives the prescribed dose while keeping the dose to the nearby OARs as low as possible. The final, approved plan contains all the geometric and dose calculation information necessary to program the linear accelerator, the machine that will deliver the radiation treatment.